The Biomarker project of the DR Program Project is concerned with (1) characterizing longitudinally the nature of changes occurring in multiple indicators of physiologic function over time in normal aging (putative biomarkers of aging) among control group ad libitum fed animals; and (2) testing how dietary restriction impacts those changes. The experimental protocol of the DR study offers a unique opportunity to examine and model individual patterns of age-related change longitudinally in a non-human primate model comprised of both males and females living in a controlled environment with uniform methods of evaluation over time. The panel of biomarkers being assessed includes multiple measures of immunologic function, body size and composition, ocular function, hematology and serum chemistry, nail growth, blood pressure, metabolic parameters including oxygen consumption and glucose regulation, endocrine markers, and skin collagen glycooxidation. Repeated measurements are obtained on individual animals on a semi-annual or annual basis and are analyzed statistically with linear and nonlinear random effects models. Recent analyses have focused on characterizing the changes observed in immunologic measures through 4 years of follow-up for the original group of 30 male monkeys. Unexpected immunological responsiveness to some standardized stimuli were reduced in restricted monkeys compared to controls. Additional work did not reveal a difference in circulating levels of thyroid hormone or in urinary metabolites of melatonin. Key words rate of aging, dietary restriction, lifespan extension